This invention relates generally to cleaning compositions for cleaning polymer compositions from polymer processing apparatus. While the principles of the invention can apply to a wide variety of polymer processing situations, the invention is described herein specifically with respect to purge compositions for purging ethylene vinyl alcohol copolymer (EVOH) from extrusion and other apparatus suitable for extruding, thus shaping or molding EVOH. Thus, the principles described herein are not limited to EVOH, but are limited only by the operability of the principles disclosed herein with respect to the polymer composition or compositions of interest.
EVOH resin is widely employed for, among other things and without limitation, food packaging and other applications as a gas and/or flavor barrier material. EVOH is usually extruded from a die in a single or multiple layer structure, and may thus be fabricated into a wide variety of shapes, such as films, sheets, or bottles.
Melted EVOH resin has a known affinity for metals, and thus tends to build up inside the extruder and downstream equipment (e.g. adapter, screens, and shaping die) of related polymer processing apparatus. EVOH is also quite sensitive to the time during which it is exposed to extruder-type temperatures. Namely, the "OH" moieties in EVOH tend to cross-link with each other.
The physical result is that, as a typical extrusion "run" progresses, viscosity of the EVOH extrudate gradually increases (MI decreases), while the amount of cross-linked EVOH builds up inside the extrusion and shaping equipment. As material continues to build up in the extrusion and shaping equipment, at some point the build-up is great enough that the material begins to break loose and show up in the extrudate as gels. The number of gels increases with time. Large numbers of gels are generally unacceptable in shaped products so made. Thus, the extruder must, from time-to-time, be purged (cleaned out) of the EVOH build-up.
One method for cleaning out the EVOH is to shut down, cool off, disassemble, and physically scrape/clean, etc. the extrusion equipment. A preferred method is to run through the extrusion and shaping equipment some material that will clean out the EVOH, including the build-up inside the respective equipment. Such material is generically known as a "purge" material, and the process is called "purging."
With EVOH, "purging" must be done periodically, even if the resin being extruded is always EVOH. Using conventional technology, the interval between "purgings" of EVOH is typically 1-7 days. Thus, "purging" is a very important part of operating an extruder even where EVOH is the only material being processed through that particular extruder.
Another situation where purging is important is where the material being extruded is to be changed. Thus, for example, if EVOH is to be extruded, followed by a run of Nylon in the same extruder, the EVOH must be fully and dependably purged from all respective polymer processing apparatus, including extruder, adapter, screen, die, and the like, before the nylon can be run. If the purging is less than complete, residual EVOH in the extruder reacts with the incoming nylon, forming an unacceptable quantity of gels in the extruded nylon polymer.
Thus, a requirement of a preferred purge material is that it must effectively remove substantially all EVOH from the polymer processing apparatus, and it must not react with the polymer composition to be subsequently extruded through that polymer processing apparatus. Nor should the purge material remain in the extruder for any extended time after the start of feeding of the subsequent polymer composition to the extruder for fabrication of product therefrom.
Transition to and from purge material is accompanied by continued operation of the extruder. Thus, the extruder screw continues to turn while changes are made in e.g. feed material, temperature settings, screw speed, and the like. Typical such transitions can be represented by sequences which look like, for example
EVOH.fwdarw.PURGE.fwdarw.EVOH PA1 EVOH.fwdarw.PURGE.fwdarw.NYLON PA1 EVOH.fwdarw.PURGE.fwdarw.POLYOLEFIN PA1 MI-mix.sub.60 /MI-mix.sub.6 of 0.3/1 to 5/1. PA1 /LDPE/PURGE/EVOH/PURGE/LDPE/. PA1 /33/7/5/7/33/.
This invention is directed toward the composition and use of the purge material, specifically as used for purging EVOH.
Materials conventionally used for purging EVOH are polyolefins such as polyethylene or polypropylene. Such purge materials can reduce build-up of EVOH in the flow channel of e.g. the extruder. However, such materials are not effective to clean the extrusion apparatus sufficiently that gel formation is reduced to an acceptable level when the extruder is subsequently used to extrude further product.
U.S. Pat. No. 3,071,498 Kaskel teaches purging polyethylene with thermoplastic granules coated with silicone oil.
U.S. Pat. No. 3,119,720 Stiles et al teach a 3-step purging process using (i) pentaerythritol, (ii) metal and a sequestering agent, and (iii) polyethylene.
U.S. Pat. No. 3,869,525 Miller teaches purging with a mixture of polyethylene and polyester.
U.S. pat. No. 4,425,288 Tieszen et al teach purging poly(arylene) sulfide from spinning equipment using polyolefin such as polyethylene or polypropylene.
U.S. Pat. No. 4,838,945 Fujii et al teach a purge composition including sodium sulfonic acid, water, and thermoplastic.
U.S. Pat. No. 4,838,948 Bailey and 4,954,545 Bailey teach purge compositions including polyethylene, sulfonated surfactant, and abrasive.
U.S. Pat. No. 5,397,498 Ishida et al teach purge compositions including polyalkylene oxide glycol and alkyl sulfate.
U.S. Pat. No. 5,424,012 Ertle et al teach using a frothing additive in a purge material.
U.S. Pat. No. 5,443,768 Scheibelhoffer et al teach purge materials including abrasive, unsaturated resin, and copolymer of olefin acrylate or methacrylate.
U.S. Pat. No. 5,460,760 Tsai et al teach coating the interior of EVOH extrusion equipment with fluoropolymer.
Japan application 5-279,518 teaches a purge material which is a combination of polyolefin and a metal composition.
Japan application 7-329,113 teaches using maleic anhydride modified polyethylene as a purge material.
Japan 62-143,954 and corresponding U.S. Pat. No. 5,032,632 teach a stabilizer package for EVOH including a metal salt and an acid component.
Japan 63-286,459 and corresponding 8-19293 teach using hindered phenol as a thermal stabilizer in EVOH.
EPA 0 308 703 A2 teaches a stabilizer package for EVOH including a metal salt, and two acid components.
None of the above resins or purge materials efficiently purge EVOH from the polymer processing apparatus. Rather, long purge times are required, and/or long run times are required of the subsequent material until the frequency of gel occurrences is low enough that the material can be used as product.
It is an object of the invention to provide an excellent cleaning composition, as a purge material, for purging EVOH and other materials from polymer processing apparatus.
It is a more specific object to provide a cleaning composition for purging EVOH, to be followed by further processing of EVOH in the same polymer processing apparatus.
It is a further specific object to provide a cleaning composition for purging EVOH, to be followed by processing of another polymer composition, devoid of EVOH, for example nylon.
It is yet another object to provide a method of purging EVOH from polymer processing apparatus.
It is still a further object to provide a method of at least temporarily maintaining uninterrupted operation of polymer processing apparatus by periodically effectively purging the polymer processing apparatus of accumulated internal build-up of polymer inside the processing apparatus.